Industrial robots (hereinafter referred to as "robots") capable of carrying various kinds of tools for automatically performing associated operations such as welding, grinding, coating, parts assembly and inspection are well known and practically used. In the field of grinding, robots provided with a grinder are used in order to automatically carry out removal of excess weld metal, finish cutting of groove faces after gas-shearing or finish cutting of the surfaces of castings. However, such robots cannot overcome the problems of over-grinding and insufficient grinding because of the possibility of variations in workpiece configuration, workpiece position and the size of objects to be removed (e.g., flash) by grinding.
There have been proposed several automatic grinding systems provided with a robot for the purpose of solving the above drawback and examples of such systems are disclosed in the following publications.
(1) Japanese Patent Laid-Open Publication No. 3-60963 (1991)
Profile grinding is performed with a grinder guided by the contour of a base metal, with the distance between the grinder and a workpiece being adjusted such that the load current for the motor for the grinder becomes constant.
(2) Japanese Patent Laid-Open Publication No. 3-142159 (1991)
A power sensor for detecting a reactive force transmitted from a workpiece is attached to the wrist of a robot. Based on information from the power sensor, the position and power of the robot are so adjusted that the pressing force exerted on the workpiece becomes constant. In addition, a set value for the pressing force is varied according to the difference between the present position and a target position.
Apart from the above-described robot systems, there are used other types of robot systems in which a single robot performs not only grinding but also a plurality of similar or the same operations on one workpiece, automatically changing tool hands according to an operation to be performed. Such systems are provided with adaptive control function for the purpose of carrying out desired operations which are impossible to cope with by the basic function (i.e., teaching playback) of the robot alone.
These prior art systems however suffer from their inherent problems. Specifically, the grinding system of the publication (1) performs grinding by the robot with constant pressing force so as to follow the contour of the base metal of a workpiece, and therefore, the shape (including flash etc.) of the object to be removed before grinding is reflected in the shape of the workpiece after grinding. This means that even though the unevenness of the workpiece can be relatively reduced, there will still remain unevenness after grinding. Such grinding systems are suited for simple polishing but unsuited for use in grinding which involves shaping operations such as removal of excess weld metal and finish cutting of castings.
The grinding system disclosed in the publication (2) needs to attach a power sensor to the wrest of the robot and therefore requires a separated processing unit for processing output data sent from the power sensor. This makes the whole system complicated.
In cases where a plurality of dissimilar operations are performed with one robot, if adaptive control is independently done for each operation, this requires different controllers for the respective operations, leading to the involvement of a large-scale control system. Furthermore, data obtained from the adaptive control for each operation needs to be individually kept in each controller, which is no more effective than the way wherein operations are separately, individually performed.
The invention has been made with a view to solving the foregoing problems and one of the objects of the invention is therefore to enable automatic grinding operation including shaping with high accuracy but without use of a complicated system, in order to grind the surface of a workpiece by use of a grinder loading robot. Another object of the invention is to make it possible to share data among a plurality of dissimilar operations which are performed with one robot, such data being concerned with each operation as well as adaptive control, so that data obtained from one equation can be utilized in other operations, which allows the whole operation to be very effective.